1. Field of the Invention
The present invention relates to a method for modifying a surface of a hydrogen storage alloy, and more particularly to a method for modifying a hydrogen storage alloy for an Ni/MH (nickel-metal hydride) secondary battery using flake type metal powder to enhance discharge capacity of an electrode and to lengthen electrode life duration.
2. Description of the Related Art
A hydrogen storage alloy refers to a metal or an alloy capable of absorbing or discharging hydrogen reversibly at a certain temperature and a certain pressure, and it must have large storage capacity of hydrogen being reversibly available and exhibit high hydrogenation velocity in an electrolyte in order to be applied in practice to an electrochemical battery.
Such a hydrogen storage alloy is used in, for example, a negative electrode of an electrochemically rechargeable battery whose positive electrode is typically made of nickel hydroxide material. A rechargeable secondary battery of this type is commonly referred to as "a nickel-metal hydride (Ni/MH) secondary battery" due to the nickel hydroxide positive electrode and the hydride nature of the negative electrode metal.
The hydrogen storage alloys for the Ni/MH secondary battery developed so far can be largely divided into three types: (1) AB.sub.5 type including La--Ni and Mn--Ni based alloys, wherein A is an element having a strong affinity for hydrogen, for example, alkali earth metal, such as, La, Ti, Zr, Ce, Pr, Nd, etc., and B is a transition element or a transition metal, such as, Ni. Mn, Co, Fe, Al, etc.; (2) AB.sub.2 type including Zr--Ni and Ti--Ni based alloys; and (3) AB type including V--Ti based alloys. The first AB.sub.5 type has a drawback in its low energy storage density while the second AB.sub.2 type is poor in nearly all of its performances. Also, in spite of its largest hydrogen storage capacity, the third AB type still has a problem in its incapability of charging/discharging in an alkali aqueous solution. Thus, in order to cope well with coming development of a high capacity and high performance Ni/MH secondary battery, it is necessary to precede researches for realizing high performances of the AB.sub.2 or AB type hydrogen storage alloys in which higher capacity is guaranteed than in the AB.sub.5 type hydrogen storage alloys.
In general, on a surface of the hydrogen storage alloy electrode is formed an oxide film which causes reduction of activated materials and acts as a barrier to the absorbance/discharge of hydrogen. This results in deterioration of every performance of alloy due to increases of contact resistance and charge transfer resistance. In the end, the oxide film becomes a major factor that reduces the discharge capacity and degrades the life duration of electrode.
To solve the above-mentioned problems, there have been developed various techniques for improving of surface characteristics of the hydrogen storage alloy, such as, an alloy design technique, an alloy surface-coating/etching technique, a surface characteristic-improving technique by means of additives, and a surface-modifying technique using a ball milling. The alloy design technique, however, has a difficulty in designing the alloy to maintain its thermodynamic characteristic, i.e., its large hydrogen storage capacity and at the same time to have excellent surface characteristics. Also, the alloy surface-coating/etching technique not only requires additional processes due to use of solutions, but is scarcely applied in practice because it is inevitably performed under a noxious atmosphere. On contrary, the surface characteristic-improving technique by means of additives and the surface-modifying technique using a ball milling are advantageous in that they can enhance all of the performances only by modifying the alloy surface without having an influence on the thermodynamic characteristic of the alloy. In connection with this, it is reported by M. A. Fetcenko et al., J. Electrochemical Society, 15 (1991) that nickel is necessary material for these surface-modifying or surface characteristic-improving techniques. Since the report, there have been proposed a technique using nickel as an additive in production of an electrode or a ball milling technique using nickel powder. Nevertheless, these surface-modifying techniques using normal type of nickel did not succeed in enhancing efficiency by lack of uniform contact of the nickel on the alloy surface.